Carcinogen Types, Testing, and Examples

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You may have heard that a substance is a carcinogen or is carcinogenic. What does this mean? How do we know that something can cause cancer?


A carcinogen is defined as something that can directly cause cancer. This can be a chemical substance, a virus, or even the medications and radiation we use to treat cancer. While many cancers are caused by a carcinogen or combination of carcinogens, the tendency to develop cancer may also be inherited as part of our genome.

Carcinogens may work in a few ways:

  • A carcinogen may directly damage the DNA in cells (cause mutations,) which in turn leads to a disruption in the normal processes of cells.
  • The carcinogen may instead cause damage and inflammation which results in the cells dividing more rapidly. When cells divide there is always a chance that a change will occur (a mutation) which in turn increases the chance of developing a cancer.


We are around carcinogens every day, whether at work, at home, or at play. Carcinogens do not cause cancer in everyone who is exposed; the ability of a carcinogen to cause cancer depends on many factors, including the amount of exposure, the length of exposure, the health of the individual, and other factors in the person's life that either raise or lower the risk of cancer.

People also differ in personal susceptibility to a carcinogen based on their genetic makeup. In many cases cancer is multifactorial, meaning that there are several factors that work together to either cause or prevent cancer. Types of carcinogens include:

  • Chemicals/substances - Certain chemicals used the home or workplace may be carcinogenic. For example, asbestos in insulation can lead to lung cancer and mesothelioma. Many people have a sense that if a chemical could cause cancer it would not be allowed in our homes. That's not the case.
  • Environmental radiation – Ultraviolet radiation from the sun is a well-known cause of skin cancer. Radon emitted from the normal decay of uranium in the soil and then trapped in homes is a leading cause of lung cancer.
  • Medical radiation - Both radiation used medically for diagnostic tests and that used to treat cancer are considered carcinogens. For example, women who receive radiation therapy after a mastectomy for breast cancer are at an increased risk of developing lung cancer due to the carcinogenicity of radiation.
  • Viruses - Viruses such as human papillomaviruses which cause oral cancer and cervical cancer, and hepatitis C which can cause liver cancer are considered carcinogens. Check out the other viruses which are thought to cause cancer.
  • Some medications - Some chemotherapy drugs and hormonal therapy can raise the risk of cancer. For example, chemotherapy used to treat women with early stage breast cancer may sometimes lead to leukemia. Oral contraceptive use may increase the risk of breast cancer in young women.
  • Lifestyle factors – Smoking and obesity are both carcinogens in that they can be responsible for the mutations which result in cancer.

Latency Period

An important to understand is the concept of a latency period. This is the time between exposure to a carcinogen and the time a cancer develops. The latency period can be very short, such as exposures to radiation in a nuclear disaster, or instead, many decades, depending upon the particular carcinogen.


It is not always easy to determine if a substance or an exposure is a carcinogen. A good example of this is smoking. It took many years of research and millions of dollars to determine the relationship of smoking to lung cancer. Many studies to evaluate substances for carcinogenicity are done on animals using high exposures. Prior to animal testing, many of these substances are first looked at in cell cultures in a lab. While it would be unethical to test substances for carcinogenicity in humans, retrospective studies looking at people with cancer, and evaluating prior exposures, are used to analyze substances or exposures to evaluate the ability to cause cancer.

Unfortunately, cell studies or animal studies cannot always tell us what will happen in human beings. What occurs with human cells in a dish in a lab may be very different than what happens given the same exposure amidst the millions of chemical reactions occurring all the time in people. Likewise, animal studies cannot always tell us what will happen with human exposure. This was the case with thalidomide, which was a safe medication in laboratory animals but caused birth defects when given to pregnant women.


There are several systems in place that define carcinogens in slightly different ways:

The Environmental Protection Agency:

  • Group A: Carcinogenic to humans
  • Group B: Likely to be carcinogenic to humans
  • Group C: Suggestive evidence of being carcinogenic to humans
  • Group D: Inadequate information to assess carcinogenicity
  • Group E: Not likely to be carcinogenic to humans

International Agency for Research on Cancer: National Toxicology Program:

  •   Group A: Carcinogenic to humans
  • Group B: Likely to be carcinogenic to humans
  • Group C: Suggestive evidence of carcinogenic potential
  • Group D: Inadequate information to assess carcinogenic potential
  • Group E: Not likely to be carcinogenic to humans

National Toxicology Program

  • Known to be carcinogenic to humans
  • Reasonably anticipated to be carcinogenic to humans

Safety Precautions

Keep in mind that not every substance that is a potential carcinogen has been tested. Not only are there millions of possible carcinogens both in nature and industry, but it’s simply not practical to test every chemical on hundreds of thousands of people (or ethical.) For that reason, it's important to practice discretion with any potential carcinogens to which you may be exposed. It's important to:

  • Read labels, and check out ingredients you are not familiar with. Some home products make a note that they contain human carcinogens. An example is some types of brass cleaner.
  • Follow directions for safe handling of chemicals at home. Read the small print on containers. Some of these recommend wearing gloves. Others recommend good ventilation, wearing a mask, or even donning a special ventilation mask (if it says this, it means that a mask is not enough to protect you). We forget that many substances can be absorbed easily through the skin. We now have patches for everything from nicotine to pain medications that take advantage of this process. A good rule of thumb is that if you wouldn't eat it, use gloves to handle it.
  • Follow recommended procedures when working with chemicals on the job. Employers are required to provide Material Safety Data Sheets on any chemicals you will be exposed to at work. Take the time to read these carefully.
  • Consider alternatives to substances with long lists of ingredients: for example, some people have found that instead of having an abundance of commercial cleaning supplies (all with ingredients that may raise your eyebrow,) you can effectively clean your home using only vinegar, lemon juice, olive oil, and baking soda. Not only does this reduce your exposure to potential carcinogens, but it's cheaper as well.


There are several databases in which you can look up chemicals and substances you are exposed to in order to determine their carcinogenicity.

A Word From Verywell

We are exposed to carcinogens in our environment every day. While it's likely there are substances we will learn are carcinogenic in the future, we can take action today to lower our exposure. Having an awareness, and recognizing that there are likely carcinogens in our environment that haven't yet been identified is a great start. Taking some simple safety precautions, such as reading labels and wearing gloves may not always be necessary, but can be wise if you aren't familiar with a product you are working with.

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Article Sources
  • Centers for Disease Control and Prevention. Occupational Cancer. Carcinogen List. Updated 04/24/17.

  • International Agency for Research on Cancer. Evaluation of Carcinogenic Risks to Humans.